Fibrous esterification of cellulose



Patented Feb. 4, 1936 UNITED STATES PATENT OFF-ICE 2,029,900 FIBROUS ESTERIFICATION OF CELLULOSE' poration of New York No Drawing. Application July 29, 1932, Serial No. 626,220

4 Claims. (01. 260-101) The present invention relates to a new procedure in the fibrous esterification of cellulose in which a portion of the bath which is employed in the pretreatment of the cellulose is removed by washing it with carbon tetrachlorid prior to the esterification.

The commonly known processes of esterifying cellulose may be divided into two main classes namely (1) those processes in which the cellulose ester formed dissolves or is dispersed in the esterification bath to form a dope, necessitating precipitation to separate the ester therefrom and (2) those processes in which the cellulose is esterified but retains its original formv so that precipitation is unnecessary. The present invention is concerned with the well known fibrous esterification processes, coming under the second class, in which an esterification bath which contains an appreciable amount of a non-solvent for the cellulose ester which is formed, is employed. If a solvent of the cellulose ester which is to be formed is employed in the pretreatment of the cellulose prior to esterification it is usually neccssary that at least a portion of such solvent be removed if theesterification is to be fibrous as otherwise the resulting cellulose ester would be in a soft, impractical condition. Obviously it it were possible to restrict thepretreatment bath of glacial acetic acid only to that amount whichmight be tolerated in the fibrous esterification bath with satisfactory results, such a condition would be ideal but such a small amount of pretreatment bath is not sufficient to thoroughly wet the cellulose in that step.

The removal of this excess amount of pretreatment liquid has usually been accomplished heretofore by pressing the liquid from the pretreated cellulose down to a certain point. For example cotton that has. been pretreated is ordinarily pressed down to about 3 times the weight 01 the original cotton prior to its fibrous esterification. The process of removing the pretreatment liquid by pressing has been found to be rather unsatisfactory and on a commercial scale the excess pre treatment liquid appears to be unevenly removed in the pressing step, so that certain portions of the cellulose may be subjected to the action of too great an amount of the pretreatment liquid during the esterification resulting in a. product which is not so desirable as that produced where the remaining pretreatment liquid is well distributed.

We have found that the pretreatment liquid in amount for a satisfactory fibrous esterification and at the same time be evenly distributed throughout the cellulose by washing the pretreated cellulose with a diluent, preferably that one which is to beemployed in the subsequent fibrous estcrification. The washing may take place once, which would usually necessitate a large amount of diluent to reduce the amount of pretreatment liquid to the desired degree or a plurality of times which would make possible the use of a smaller amount of diluent for each washing, which would probably be preferred in practice. Countercurrent washing may be employed if desired in removing pretreatment liquid from the cellulose according to our process.

A specific example illustrative of the application of our invention to a fibrous esterification process is as follows:

50 grams of cotton linters were pretreated with a mixture of 350 grams of glacial acetic acid and 2 cc. of a catalyst consisting of 1 part of sulfuric and 3 parts of phosphoric acids for about 15-20 hours at room temperature (-75 F.) At the end of this time 250 c. c; of carbon tetrachlorid were added to the mass and after the mixture had stood a few minutes, it was allowed to drain by gravity until 220 c. c. of the liquid were recovered. Analysis of this drainage liquor showed that grams of acetic acid had been removed.

250 c. c. of carbon tetrachlorid were added to the pretreated cotton and 240 c. c. of liquid were allowed to drain ofi in the same manner as before. This washing, step removed 89 grams of acetic acid. c. c. of carbon tetrachlorid were then added and 150 'c. c. of liquid were drained off removing 33 grams more of acetic acid. Thus a total of 252' grams of acetic acid were removed from the cotton leaving 98 grams remaining on the cotton which is approximately the amount required.

The total weight of the remaining mass was 515 grams of which 50 grams were cotton, 98 grams were acetic acid and 365 grams of carbon tetrachlorid. These proportions are approximately the same as those of these same ingredients in the usual procedure employed for the fibrous esterification of cellulose. Acetic anhydrid was then added to the mass and the acetylation was carried out in the usual way.

As pointed out above the pretreated cellulose may be washed by a counter current method with a diluent such as carbon tetrachlorid according to our invention, it it is desired. In this countercurrent washing of the pretreated cellulose either that material may be moved from one tank to other tanks of decreasing concentrations of acetic acid in carbon tetrachlorid or the cellulose may be allowed to remain in the same tank and be subjected to successive washings of progressively diminishing concentrations of acetic acid in carbon tetrachlorid. The latter method of countercurrentwashing may be carried out in a manner similar to that disclosed in application Serial No. 540,108 of Messrs. G. J. and H. T. Clarke, filed May 26, 1931.

This counter current washing may be carried out with the requisite number of steps to assure the presence of only the amount of acetic acid which may be tolerated in the subsequent fibrous acylation step. Due to the difference between the boiling points of carbon tetraohlorid and acetic acid the former may easily be distilled off from the latter especially in the case of the mixtures obtained from the first step in the countercurrent method in which a predominating proportion of acetic acid is present. In this way glacial acetic acid suitable for use in the pretreatment of subsequent cellulose is easily obtained. This method, as is at once apparent to the skilled technician, offers a decided improvement over that of pressing which has been employed heretofore especially where the handling of large quantities of the cellulose is necessary.

Although the invention is described with cotton as the cellulosic material employed other cellulosic materials which are suitable for fibrous esterification may be employed in the process we have invented. Sulfite wood pulp or cellulose derivatives such as cellulose acetate, etc. which admit of esterification may be employed as the starting material of our process.

Other pretreating liquids instead of acetic acid and other diluents than carbon tetrachloride may also be employed according to the judgment of the individual operator without departing from the scope of our invention. For example it has been known to employ, instead of acetic acid, in the pretreatment of cellulose, mixtures of acetic acid with other liquids such as ethylene chloride, propionic acid, nitric acid, etc., or some of these liquids may be used for pretreating the cellulose without the acetic acid.

The diluent which is used to wash out the pretreatment liquid is generally the same diluent which is to be employed in the esterification. This is however merely customary procedure and it is understood that one diluent may be employed for washing out the pretreatment liquid and a different diluent may be added for the esterification step providing these diluents are compatible. Any of the well known diluents may be employed in our process such as ether, benzine, benzene, toluene, etc.

Our invention obviously may be employed in making the well-known cellulose acetate as illustrated by the example or in the production of the higher and mixed esters of cellulose. For example propionic acid or acetic acid may be em-.

ployed in the pretreatment and either the anhydride not corresponding to the acid there employed or a mixture of propionic and acetic anhydrides may be employed in the esterification to form cellulose acetate propionate. As pointed out above a cellulose acetate having free and available hydroxyl groups may be propionated to form the mixed ester of cellulose. Other acids or anhydrides may be used instead of propionic in any of the above procedures to form the corresponding ester of cellulose.

Various other modifications which are obviously the result of the skill and technique of the individual operator also come within the scope of our invention.

We claim as our invention:

1. In the process of preparing a cellulose ester in fibrous form in which the cellulose is pretreated with an acid and subsequently acylated in a bath containing a predetermined amount of said acid, of a fatty acid anhydride, and of an inert diluent; the step which comprises reducing the acid content of the pretreated cellulose to said predetermined amount and simultaneously providing the predetermined amount of diluent for the acylation bath by washing the pretreated cellulose with a liquid which is a non-solvent for the fibrous ester.

2. In the process of preparing a cellulose ester in fibrous form in which the cellulose is pretreated with an acid and subsequently acylated in a bath containing a predetermined amount of said acid, of a fatty acid anhydride, and of carbon tetrachloride; the step which comprises reducing the acid content of the pretreated cellulose to said predetermined amount and simultaneously providing the predetermined amount of carbon tetrachloride for the acylation bath by washing the pretreated cellulose with a liquid which is a non-solvent for the fibrous ester.

3. In the process of preparing a cellulose ester in fibrous form in which the cellulose is pretreated with acetic acid and subsequently acylated in a bath containing a predetermined amount of.

acetic acid, of a. fatty acid anhydride, and of an inert diluent; the step which comprises reducing the acetic acid content of the pretreated cellulose to said predetermined amount and simultaneously providing the predetermined amount of diluent for the acylation bath by washing the pretreated cellulose with a liquid which is a non-solvent tor the fibrous ester.

4.In the process of preparing a cellulose ester in fibrous form in which the cellulose is pretreated with acetic acid and subsequently acylated in a bath containing a predetermined amount of acetic acid, of a fatty acid anhydride, and of carbon tetrachloride; the step which comprises reducing the acetic acid content of the pretreated cel lulose to said predetermined amount and simultaneously providing the predetermined amount of carbon tetrachloride forthe acylation bath by washing the pretreated cellulose with a liquid which is a non-solvent for the fibrous ester.

RUSSEL H. VAN DYKE.

CYRIL J. STAUD. HARRY LE B. GRAY. 

